1. Field of the Invention
This invention lies in the field of glass preforms for drawing fiber optical waveguides for optical communications technology.
2. Prior Art
A major goal in light waveguides for use on optical communications technology is to achieve the lowest possible attenuation and pulse spread. Good homogeneity of the glass comprising a waveguide is a prerequisite for a low attenuation and a certain radial profile of refraction is a prerequisite for the low pulse spread. The profile of a paraboloid is suitable as the refractive index profile. Given this profile, it is possible to largely compensate for differences of transit time between individual mode groups in multimode glass fibers, and, thus, to keep pulse spread low. Thereby, the refractive index profile must be very precisely observed.
Glass fibers with a desire refractive index profile can be manufactured in such a manner that a glass rod is first manufactured which exhibits a corresponding refractive index profile. The fibers are drawn from such rod in such a manner that the shape of the refractive index profile in the rod is retained in the fibers drawn therefrom.
Glass rods with refractive index profiles can be manufactured in a method of the type initially cited. The so-called CVD method which is included among the methods of the type initially cited is particularly suited for step-shaped radial refractive index profiles such as exist, for example, given core-jacket glass fibers with a constant refractive index in the core and a constant refractive index in the jacket. In a current method of this type, the glass layers are deposited on the inside wall of a glass tube and the tube thus coated on the inside is deformed (melted) into a rod from which the desired fibers can be drawn.
In order to so deposit the glass layers, a reactive gas mixture is conducted through the tube which is heated from the outside. The chemical reaction is thermally triggered in the inside in the heating zone, whereby a powder exhibiting the composition of a desired glass is produced, is deposited on the inside wall of the tube, and is clear-melted into a glass film in the heating zone. The refractive index of the deposited glass can be influenced by the composition of the reactive gas mixture. Glass rods with a step-shaped refractive index profile, particularly for core-jacket glass fibers which are manufactured with the CVD method, are distinguished by a high homogeneity of the glass and the fibers drawn from them are distinguished by a low attentuation.
Refractive index profiles with a continuous curve, which, for example, correspond to a paraboloid profile, can be achieved with the CVD method by means of depositing a multitude of glass layers with refractive indexes which differ slightly from one another.
Thereby, in the production of a desired refractive index profile with a continuous curve, particularly having the profile of a paraboloid, it has turned out that settle marks, or ripples, occur of such a type that the individual layers no longer exhibit the desired homogenous composition in radial direction. It has further turned out that it is not practical to eliminate such inhomogeneities like settle marks or ripples by means of increasing the plurality of glass layers to be deposited while reducing the layer thicknesses.
Moreover, the preforms produced with such a known method exhibit a central disruption of the refractive index, particularly a central refractive index drop which has a disadvantageous influence on the band width of the fiber optical waveguides drawn from such a preform. This disruption has an increasingly disadvantageous effect on the band width with increases in breadth of the central disruption zone. A discussion of the "Effects of profile deformations on fiber bandwidths" was given by D. Marcuse and H. M. Presby in Appl. Opt. 18(1979) pp. 3758-3763.